calculate-pi-using-sql.md

date
2023-03-14

It's Pi Day! Let's calculate pi using SQL

Happy Pi Day! We thought it would be fun to calculate pi using SQL queries in ClickHouse. Here is what we came up with so far...

1. This one uses the ClickHouse numbers_mt table function to return 1B rows and only takes 40ms to compute the calculation:

SELECT 4 * sum(if(number % 2, -1, 1) / ((number * 2) + 1)) AS pi
FROM numbers_mt(1000000000.)

┌────────────────pi─┐
│ 3.141592652589797 │
└───────────────────┘

1 row in set. Elapsed: 0.432 sec. Processed 1.00 billion rows, 8.00 GB (2.32 billion rows/s., 18.53 GB/s.)

2. The following example also processes 1B numbers, just not as quickly:

SELECT 3 + (4 * sum(if((number % 2) = 0, if((number % 4) = 0, -1 / ((number * (number + 1)) * (number + 2)), 1 / ((number * (number + 1)) * (number + 2))), 0))) AS pi
FROM numbers_mt(2, 10000000000)

┌─────────────────pi─┐
│ 3.1415926525808087 │
└────────────────────┘

1 row in set. Elapsed: 9.825 sec. Processed 10.00 billion rows, 80.00 GB (1.02 billion rows/s., 8.14 GB/s.)

3. This one is obviously our favorite in ClickHouse (and the most accurate!):

SELECT pi()

┌──────────────pi()─┐
│ 3.141592653589793 │
└───────────────────┘

1 row in set. Elapsed: 0.008 sec.

4. Someone knew their trigonometry with this one:

SELECT 2 * asin(1) AS pi

┌────────────────pi─┐
│ 3.141592653589793 │
└───────────────────┘

1 row in set. Elapsed: 0.005 sec.

5. Here is a handy API that lets you specify the number of digits you want:

SELECT *
FROM url('https://api.pi.delivery/v1/pi?start=0&numberOfDigits=100', 'JSONEachRow')

┌───────────────content─┐
│ 3.1415926535897933e99 │
└───────────────────────┘

1 row in set. Elapsed: 0.556 sec.

6. This one is clever - it uses ClickHouse distance functions:

WITH random_points AS
    (
        SELECT (rand64(1) / pow(2, 64), rand64(2) / pow(2, 64)) AS point
        FROM numbers(1000000000)
    )
SELECT (4 * countIf(L2Norm(point) < 1)) / count() AS pi
FROM random_points


┌──────────pi─┐
│ 3.141627208 │
└─────────────┘

1 row in set. Elapsed: 4.742 sec. Processed 1.00 billion rows, 8.00 GB (210.88 million rows/s., 1.69 GB/s.)

7. If you're a physicist, you will be content with this one:

SELECT 22 / 7

┌─────divide(22, 7)─┐
│ 3.142857142857143 │
└───────────────────┘

8. Another indirect mehthod (this one came from Alexey Milovidov) that is accurate to 7 decimal places - and it's quick:

WITH
    10 AS length,
    (number / 1000000000.) * length AS x
SELECT pow((2 * length) * avg(exp(-(x * x))), 2) AS pi
FROM numbers_mt(1000000000.)


┌─────────────────pi─┐
│ 3.1415926890388595 │
└────────────────────┘

1 row in set. Elapsed: 1.245 sec. Processed 1.00 billion rows, 8.00 GB (803.25 million rows/s., 6.43 GB/s.)

:::note If you have any more, we'd love for you to contribute. Thanks! :::